The present invention relates to a conveyance system for powdery or granular material which is transported in a dense stream by means of compressed air fed into the pipe line.
It is known that a minimum flow rate must be exceeded in pneumatic conveyance pipes, otherwise, below this so-called blockage limit the resistance due to the material in the pipe becomes so large that the pneumatic pressure is no longer adequate to move the material.
One method which has been adopted to prevent possible blockage of the pipe is to introduce a porous pipe of smaller diameter into a pipe which is impermeable to air. The remaining, free space between the pipes is filled with compressed air to loosen up the material being transported. This allows the blocking tendency to be reduced, however, a minimum flow rate, even though a reduced rate, must still be exceeded.
A device to remove powdery or granular material from a horizontal or inclined supply pipe has been suggested whereby the material is conveyed by compressed air from a lower into an upper, parallel pipeline. Both pipelines for the material and for the compressed air are connected via a porous base which extends basically over the whole length of the pipe. The air is able to pass through this porous base and maintain the loose character of the material.
This device has, however, the disadvantage that the greater part of the compressed air follows the path of least resistance and enters, at the end of the lower compressed air pipeline, into the upper pipe containing the material where the resistance is several orders of magnitude greater than in the empty lower pipeline. On the other hand at the start of the feed pipe or in the middle stretch almost no compressed air from the lower pipe enters the upper pipe, which is a disadvantage when trying to achieve a dense flow of material. Dense flow is not defined as a number itself but as a ratio where ##EQU1## which as a rule lies above a value of 50.
Dense flow transportation is of great economic importance, as the alternative viz., dilute flow requires the material being transported to reach a high flow rate in the pipe in order to achieve a reasonable level of throughput. This, in turn, results in high energy consumption and high wear rates.
It is therefore a principal object of the present invention to develop a feed pipe system which allows powdery or granular material to be transported in a dense stream with a minimum expenditure of energy and such that the wear on the feed pipe will be so slight that the pipe will have a service life of several years. The feed pipe system should be conceived in such a way that at the end of the feeding process, the pipe does not have to be blown empty.